With a communication system including a wireless communication system and a memory, an oscillator whose oscillation frequency is variable is an indispensable circuit. Along with progress in the development of the communication system, an oscillator with an oscillation frequency close to a GHz range has lately come into use. In the case of making up an oscillator by use of an LC resonator comprised of an inductor L and a capacitor C, inductance and capacitance become small at such a high frequency, so that the oscillator can be easily formed on a semiconductor substrate through integration with a transistor. In this case, for the inductor, use is made of a thin line formed in a shape square or circular, or in a spiral shape, and so forth while as the capacitor, use is made of a pn-junction capacitor (diode capacitor), a gate-source-drain capacitor (hereinafter referred to as “a MOS capacitor”) of a MOS (Metal Oxide Semiconductor) transistor, a MIM (Metal Insulator Metal) capacitor formed between metal layers in a semiconductor device, and so forth. The structure and operation of the MOS capacitor have been disclosed in, for example, Non-patent Document 1: “Understanding Semiconductor Devices” by Sima Dimitrijev, published by Oxford University Press, in 2000, pp. 121 to 129.
Variation in frequency is executed by changing a control voltage applied to the diode capacitor and the MOS capacitor, and changing over among a plurality of the MIM capacitors, and by other means, and further, for the purpose of obtaining a wide frequency-tuning range, varying inductance is also executed.
As a technology for rendering inductance of integrated inductors variable, a technology for switching over inductance by switching over a whole length of the inductor by use of a switch using a linear region of a MOS transistor has been disclosed in, for example, Non-patent Document 1. Further, Patent Document 1; Japanese Patent Laid-open No. 2002-151953, has disclosed a technology whereby an inductor is made up of a primary inductor and a secondary inductor magnetically coupled thereto, and presence/lack of mutual inductance applied to the primary inductor is controlled by opening/closing the secondary inductor by use of a switch to thereby control inductance of the primary inductor. With the technology disclosed in Patent Document 1, control is made merely by opening/closing the secondary inductor, so that inductance is selectable for two values only, however, Patent Document 2; Japanese Patent Laid-open No. 2002-280222, has disclosed a variable inductor capable of selecting a plurality of inductances by selectively closing a plurality of open circuits each with a single loop winding. Further, Patent Document 3; Japanese Patent Laid-open No. 2004-140165, has disclosed a technology whereby an inductor is made up of a first inductor and a second inductor formed so as to have mutual inductance with the first inductor as with the case of Patent Documents 1, and 2, thereby rendering inductance variable by varying resistance of a variable resistor connected in series to the second inductor. Still further, Patent Document 4; Japanese Patent Laid-open No. 2002-9544, has disclosed a technology whereby a control inductor is disposed in the vicinity of an inductor used for an oscillator, and mutual inductance between both the inductors is varied by causing a control current to flow to the control inductor